1. Field of the Invention
The present invention is related to character generating sytems and, more particularly, is directed towards a character generator of the raster scan type which displays a selected character as a dot matrix, and means for varying the size and position of the selected character.
2. Description of the Prior Art
A conventional character generator employed both in television broadcasting and in video terminals digitally generates characters for display by utilizing a read-only memory (ROM) which permanently stores the dot pattern or matrix for each character of a predetermined set of characters or symbols. For example, ROM integrated circuit chips are available which store a set of sixty-four characters, each character consisting of a 5.times.7 dot matrix. Other somewhat higher resolution ROM dot pattern matrices include 7.times.9 dots and 7.times.11 dots. For high resolution, such as is required in broadcast television, a ROM character generator having a 32.times.32 dot pattern matrix for each character may be utilized.
A standard technique for generating visually observable signals manifesting the characters stored in a ROM is to address simultaneously the ROM by a first multibit character code, such as the well-known ASCII code, and a second multibit scan line code. The character code defines the particular one of the set of characters to be read out from the ROM, while the scan line code defines the particular consecutive scan line of the character then being scanned. In response to these two multibit codes, the ROM loads the various stages of a shift register with the pattern of dot signals that correspond to the particular character and scan line then being read out. This loading of the shift register occurs at the onset of each charactor display interval. Subsequently during the same interval, the pattern of dot signals is shifted out serially by a dot clock to produce the digital video. It may be appreciated, therefore, that the shift register serves as a parallel-to-serial converter. The video signal output from the shift register, after being combined with suitable sync and blanking signals, may then be employed either directly, e.g. in a video terminal, or indirectly, e.g. in television broadcasting, as an intensity-modulating Z-axis signal of a television cathode ray tube.
The size and shape of the set of characters stored in such a ROM character generator is, by its very nature, fixed and permanent. The baseline position of the displayed characters on the screen is also generally predetermined.
From a user's point of view, it would be highly desirable to have the capability of generating high resolution characters from a fixed ROM set of characters which may be selectively varied in height, width and position relative to other characters on the screen. It would further be highly desirable if such size varying capabilities were available on a character-by-character basis. It is towards achieving these objectives that the present invention is advanced.
I am aware of several prior art techniques and systems which have been utilized to vary character size in a read-only memory character generator.
For example, the Model D-3000 television character generator manufactured by Datavision Video Products of Gaithersburg, Md., employs a shift key on the keyboard which serves to select either a large character typing mode or a small character typing mode. The output signal developed by this shift key is fed on a single bit line as a character size signal to an auxiliary read-only memory which also receives as inputs the scan line code. The outputs of this auxiliary ROM consist of scan line signals which have been modified depending upon the single character size bit of information. The modified scan line signals are then delivered to the fixed size character ROM set and serve to select one of two modes for reading out the lines of dot signals. This technique, therefore, permits a user to select one of two different heights for the character selected to be displayed. While an improvement over the prior art, this system is limited in versatility in that only one character height different from normal full height can be selected. Further, the characters are all of constant width, regardless of the height selected, and all characters are located on an invariable baseline.
I am also aware of U.S. Pat. No. 3,754,229 which provides proportional spacing of the characters on the screen by controllably varying the speed at which the dot signals are generated during uniform scans of the screen. While being an improvement over the constant width character generators of the prior art, the system described in this patent also lacks versatility in that the width for each character is predetermined. That is, while some characters may be wider than others, no means are provided for permitting the user to select a particular width desired for a particular character. Further, the system described in this patent does not provide for any height or baseline variation.
Other United States patents in the character generation art of which I am aware include: U.S. Pat. Nos. 3,659,283; 3,816,823; 3,893,100; 3,928,845; 4,053,878; 4,081,799; 4,090,188; 4,107,662; 4,107,665; 4,107,786; 4,119,954; 4,121,228; and 4,129,859. Several of the patents, such as U.S. Pat. Nos. 4,081,799 and 4,090,188, are primarily involved with increasing the size of the generated characters for increasing resolution, and employ complex algorithms for achieving this result. Such systems are of extremely limited usefulness and versatility vis-a-vis a truly variable height, width and position character generator.
The aspect ratio of a displayed character, defined as the ratio of its width to its height, is frequently of more significance to the user of a video character generator than height or width information by itself. That is, a printer or operator may feel more comfortable specifying the desired size of a character by specifying its aspect ratio and either its height or width than by simply specifying the height and width separately. This is due to the common practice in the printing art of specifying the size of a character font by its aspect ratio and height or width. It can therefore further be appreciated that it would be highly desirable if inputs to a keyboard for a character display system of the type discussed above could be provided which permitted a user to specify character size by simply specifying a selected height or width and a desired aspect ratio for the character, rather than selecting the height and width independently. It is also toward achieving this object that the present invention is advanced.